1. Field of the Invention
The present invention relates to a terahertz wave temporal waveform acquisition apparatus.
2. Related Background Art
A terahertz wave is an electromagnetic wave having a frequency of about 0.01 THz to 100 THz corresponding to a region in the middle of a light wave and a radio wave, and has an intermediate nature between the light wave and the radio wave. As an application of the terahertz wave, research has been conducted on a technique for obtaining information on a measurement object by measuring a temporal waveform of an electric field amplitude of the terahertz wave passed through or reflected by the measurement object.
A general technique for measuring the information on the measurement object by using the terahertz wave is as follows. That is, pulsed light output from a light source (such as femtosecond laser light source) is branched by a branch part into two beams, i.e., pump light and probe light. The pump light is input to a terahertz wave generation element (for example, nonlinear optical crystal or photoconductive antenna element), and according to this, the terahertz wave generation element generates the pulsed terahertz wave. The generated terahertz wave obtains the information on the measurement object (for example, absorption coefficient, refractive index) by passing through or being reflected by the measurement object, and after that, the terahertz wave is input to a terahertz wave detection element (for example, electrooptical crystal or photoconductive antenna element) at the substantially same timing as the probe light. The terahertz wave detection element to which the terahertz wave and the probe light have been input detects a correlation value between the terahertz wave and the probe light.
When the electrooptical crystal is used as the terahertz wave detection element, the terahertz wave and the probe light are coupled by a coupling part and input to the electrooptical crystal, a birefringence is induced in the electrooptical crystal along with propagation of the terahertz wave, and the birefringence changes a polarization state of the probe light. The change of the polarization state of the probe light in the electrooptical crystal is detected, and thus the electric field amplitude of the terahertz wave is detected and the information on the measurement object is obtained.
Further, when the photoconductive antenna element is used as the terahertz wave detection element, a current expressing the correlation value between the terahertz wave and the probe light is generated between two electrodes of the photoconductive antenna element according to the inputs of the terahertz wave and the probe light. A spectrum of the terahertz wave is obtained, based on the correlation value, and in addition, the information on the measurement object is obtained.
Generally, the pulse width of the terahertz wave is about a picosecond, whereas, the pulse width of the probe light is about a femtosecond, and the pulse width of the probe light is narrower than that of the terahertz wave by several digits. According to this, the input timing of the terahertz wave or the probe light to the terahertz wave detection element is swept so that the temporal waveform of the electric field amplitude of the terahertz wave is obtained.
Non-Patent Document 1: P. Y. Han and X.-C. Zhang, “Free-space coherent broadband terahertz time-domain spectroscopy”, Meas. Sci. Technol. 12, pp. 1747-1756 (2001)